Sheet feeding apparatus and image forming apparatus

ABSTRACT

A sheet feeding apparatus including: a separating roller which abuts against a feed roller and is rotated in association with the feed roller; and a torque-limiter for allowing the separating roller to rotate in association with the feed roller when a rotation torque larger than a predetermined torque is acted on the separating roller and for preventing the separating roller from rotating in association with the feed roller when the rotation torque equal to or smaller than the predetermined torque is acted on the separating roller, in which sheets fed from a sheet feed cassette are separated one by one by the feed roller and the separating roller, the separating roller urged against the feed roller is supported by a guide member in a slidable manner, and the separating roller is guided with a predetermined angle in a direction different from a direction in which the feed roller is opposed to the separating roller.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet feeding apparatus and an imageforming apparatus, and more particularly, to a structure of a sheetseparating means for separating sheets fed by a sheet feeding means.

2. Related Background Art

Conventionally, an image forming apparatus such as a printer, a copyingmachine, and a facsimile is provided with sheet feeding apparatus forseparating the sheets one by one to feed the sheets from a sheetstacking portion, on which plural sheets of recording paper or originals(hereinafter referred to as “sheets”) are stacked, to an image formingportion.

As a sheet separating means of the sheet feeding apparatus describedabove, one involving a separation method using a separating pad isknown. In such the separation method using the separating pad, in a casewhere plural sheets are fed between a feeding roller and the separatingpad, sheets other than an uppermost sheet are dammed up by an action offriction separation caused between the feeding roller and the separatingpad. It should be noted that a feature of the sheet separating meansusing such the separating pad resides in that a sheet separating portioncan be constructed to be small in area.

Next, a detailed structure of such the sheet separating means using suchthe separating pad with reference to the drawings. FIG. 19 is aschematic diagram showing a structure of a printer adopting a separatingpad as the sheet separating means.

In FIG. 19, reference numeral 101 denotes a printer main body(hereinafter referred to as “apparatus main body”), reference symbol101A denotes an image forming portion. Below the image forming portion101A, 2-stages of sheet feeding portions 102 and 106 constructed of anupper sheet feeding portion 102 and a lower sheet feeding portion 106,are arranged so as to overlap with each other. The image forming portion101A is provided with a laser scanner 114, an image forming process unit113 including a photosensitive drum 113 a serving as an image bearingmember, a transfer roller 113 b constituting a transferring portion fortransferring a toner image formed on the photosensitive drum 113 a ontoa sheet S, and so on.

Reference numeral 115 denotes a fixing device for fixing the toner imagetransferred by the transferring portion onto the sheet. After the tonerimage is fixed by the fixing device 115, the sheet S is sequentiallydischarged to and stacked on a discharge tray 119 provided on anuppermost portion of the apparatus main body. Further, reference numeral131 denotes an option sheet feeding apparatus provided with a sheet feedcassette 131A, and the option sheet feeding apparatus 131 is selectivelymounted on a bottom surface of the apparatus main body 101.

As shown in FIG. 20, the upper sheet feeding portion 102 includes anupper sheet feed cassette 105A, an upper feed roller 103 for feeding outsheets S stored in the upper sheet feed cassette 105A, and an uppersheet separating portion 104 provided with an upper separating pad 104 afor separating the sheets S fed from the upper feed roller 103 one byone.

It should be noted that reference symbol 104 b denotes an upperseparating pad supporting portion for bearing the upper separating pad104 a, and is provided to the upper sheet separating portion 104 in arotatable manner. The upper separating pad supporting portion 104 b isurged against the upper feed roller side by an upper separating padspring 104 c so as to pressure-contact with the upper feed roller 103.

In addition, reference symbol 105 a denotes an upper sheet stackingplate for stacking the sheets S, and is provided to the upper sheet feedcassette 105A in a rotatable manner. The upper sheet stacking plate 105a is pressed upwardly by an upper sheet stacking plate pressure spring105 b from a back surface side (downside in FIG. 20) of the upper sheetstacking plate 105 a. A leading end of the uppermost sheet S1 stacked onthe upper sheet stacking plate 105 a is pressed against the upper feedroller 104.

In the upper sheet feeding portion 102 with this structure, the upperfeed roller 103 rotates counterclockwise by a drive of a driving motor(not shown) and conveys the uppermost sheet S1, which is stacked on theupper sheet stacking plate 105 a, between the upper feed roller 103 andthe upper separating pad 104 a. At this time, in a case where pluralsheets S are conveyed between the upper feed roller 103 and the upperseparating pad 104 a, sheets other than the uppermost sheet S1 aredammed up by the friction separating effect caused by the upperseparating pad 104 a, and only the uppermost sheet S1 is conveyed to adownstream side. It should be noted that the lower sheet feeding portion106 and an option sheet feeding portion 132 of the option sheet feedingapparatus 131 are constituted in a similar manner as the upper sheetfeeding portion 102.

In FIG. 20, reference numeral 120 denotes a conveying path for an optionsheet feeding apparatus for conveying the sheet S fed from the optionsheet feeding apparatus 131 upwardly (downstream side), the conveyingpath 120 for the option sheet feeding apparatus merges into a conveyingpath for a lower sheet feeding portion 121 for conveying the sheet S fedfrom the lower sheet feeding portion 106 upwardly (downstream side).

Further, a conveying path 122 for the upper sheet feeding portion forconveying the sheet S fed from the upper sheet feeding portion 102upwardly (downstream side) merges into the conveying path 121 for thelower sheet feeding portion. Thus, the sheet fed from the option sheetfeeding apparatus 131, the lower sheet feeding portion 106, and theupper sheet feeding portion 102 passes through a sheet feeding portionmerging conveying path 123 to be conveyed to the downstream side.

Then, the sheet S which has passed through the sheet feeding portionmerging conveying path 123 to be conveyed to the downstream side is, insynchronization with an image formed on the photosensitive drum 113 a bya registration roller 112 shown in FIG. 19, conveyed to the transferringportion constituted of the photosensitive drum 113 a and a transferroller 113 b. In the transferring portion, a toner image is transferredonto the sheet S.

It should be noted that the sheet S onto which the toner image has beentransferred is then conveyed to the fixing device 115, and the tonerimage is fixed on the sheet S by applying a pressure and heat at thefixing device 115. After that, the sheet S is conveyed through a pair ofconveying rollers 116 and 117 and a pair of discharge rollers 118 to bedischarged and stacked sequentially on the discharge tray 119.

By the way, according to the sheet separating means adopting such theseparation method using the separating pad, the following structure canbe employed by taking an advantage of being capable of constructing thesheet separating portion within a small space.

In other words, in a conventional sheet feeding apparatus, as shown inFIG. 20, since a bottom surface of the upper sheet feed cassette 105A ofthe upper sheet feeding portion 102 is overlapped with the lower sheetfeeding portion 106, the height of the apparatus main body 101 can bereduced by the overlapped portion. In addition, since the sheetseparating portion requires only a small space, a lower feed roller 107of the lower sheet feeding portion 106 can be provided in adjacent tothe upper sheet separating portion 104 side. As a result, a compactproduct can be realized in which the height of the apparatus is madelow, while suppressing the width.

Further, in such a type of printer that the option sheet feedingapparatus 131 is to be added, when the conveying path 120 for the optionsheet feeding apparatus is arranged so as to be adjacent to a lowersheet separating portion 108, the lower sheet separating portion 108requires only a small space, thereby making it possible to obtain acompact product capable of suppressing the apparatus width. Therefore,with this structure, a compact product, which allows a printer forA3-size paper to be arranged on a desktop, is known.

On the other hand, as disclosed in JP 62-105834 A, a sheet separatingmeans adopting a separating roller with a torque-limiter function isknown as another example of the sheet separating means to be mounted tothe sheet feeding apparatus. It should be noted that in such the sheetseparating means, the separating roller, which is positioned in coaxialwith the torque-limiter or is built in the torque-limiter, ispressure-contacted with the feed roller, to thereby separate the sheetsby using a braking torque of the torque-limiter.

For example, when only one sheet is placed between the feed roller andthe separating roller, a large rotation torque is acted on thetorque-limiter, thereby the torque-limiter allows the separating rollerto rotate in association with the feed roller. On the other hand, whenplural sheets are placed between the feed roller and the separatingroller, a relatively small rotation torque is acted on thetorque-limiter, thereby the torque-limiter prevents the separatingroller from rotating in association with the feed roller. As a result,the feed roller conveys one sheet and the separating roller rotates inan opposite direction from the feed roller, to thereby prevent anothersheet from being conveyed.

Thus, the braking torque of the torque-limiter controls the brakingtorque within a range from a lower limit, at which the torque-limiterprevents the separating roller from rotating in association with thefeed roller when plural sheets locate between the feed roller and theseparating roller, to an upper limit, at which the torque-limiter allowsthe separating roller to rotate in association with the feed roller whenonly one sheet is placed between the feed roller and the separatingroller, to thereby obtain a sheet separating function and a sheetfeeding performance. Then, with this structure, it is known that, ascompared with the sheet separating means using the separating pad, thesheet separating means with high durability, without strange sound(noise) cause by chattering marks of a pad and a sheet, and capable ofperforming stable sheet feeding can be obtained.

Next, a detailed structure of the sheet separating means provided withsuch the separating roller will be described.

FIG. 21 shows a schematic diagram for explaining a structure of aprinter mounted with the sheet separating means provided with aseparating roller using a torque-limiter. It should be noted that inFIG. 21, the same reference symbols as those of FIG. 19 indicate theidentical or corresponding portions.

In FIG. 21, reference numeral 153 denotes a sheet feeding portion, thesheet feeding portion 153 is constituted of a sheet feed cassette 152, apickup roller 154 for feeding out the sheet S stored in the sheet feedcassette 152, a feed roller 155, and a separating roller 156 a beingpressure-contacted with the feed roller 155. The sheet feeding portion153 includes a separating portion 156 for separating the sheets S fed bythe pickup roller 154 one by one.

It should be noted that the separating roller 156 a, as shown in FIG.22, is rotatably held at a rotating end of a supporting member 156 cwhich is supported rotatably about a rotation axis 156 d, and is pressedagainst the feed roller 155 by a spring 156 e. In addition, theseparating roller 156 a is coupled onto a rotation axis 156 b through atorque-limiter (not shown).

Further, reference symbol 152 a denotes a sheet stacking plate which isrotatably provided to the sheet feed cassette 152. The sheet stackingplate 152 a is pressed upwardly by a sheet stacking plate pressurespring 152 b from a back surface side (downside in FIG. 22) of the sheetstacking plate 152 a, and a leading end of the uppermost sheet S1stacked on the sheet stacking plate 152 a is pressed against the pickuproller 154.

Then, in the sheet feeding portion 153 with this structure, the pickuproller 154 and the feed roller 155 rotate counterclockwise by a drive ofa driving motor (not shown) and convey the uppermost sheet S1, which isstacked on the sheet stacking plate 152 a, to a nip portion between thefeed roller 155 and the separating roller 156 a. Then, in the nipportion between the feed roller 155 and the separating roller 156 a, thesheets are separated by a braking torque of the torque-limiter.

By the way, as the sheet separating means with this structure, asdisclosed in JP 2003-02634 A, there is known a sheet separating meansfor feeding and separating sheets by one roller having a large diameterwhich serves as a pickup roller and a feed roller in common and forconveying the sheets to an image forming portion or an image readingportion. With this structure, since the number of parts can be reduced,thereby making it possible to reduce the cost, minimize a space for thesheet separating portion, and to downsize the apparatus.

FIG. 23 is a diagram for explaining an example of the sheet separatingmeans which can be used commonly as a pickup roller and a feed roller.In FIG. 23, reference numeral 175 denotes a feed roller having a largediameter which can be used as a pickup roller. In addition, referencesymbol 176 a denotes a separating roller, the separating roller 176 abeing rotatably held at a rotating end portion of a supporting member176 c which is supported rotatably about a rotation axis 176 d, and ispressure-contacted with the feed roller 175 by a spring 176 e. Theseparating roller 176 a is, similarly to the above conventional art,coupled onto a separating roller rotation axis 176 b through atorque-limiter (not shown).

The feed roller 175 is pressed against a leading end of the uppermostsheet S1 stacked on the sheet stacking plate 152 a, and is pressedagainst the separating roller 176 a at a downstream side in acircumferential direction. At the nip portion between the feed roller175 and the separating roller 176 a, the sheets are separated by thebraking torque of the torque-limiter.

In the sheet separating means adopting such the separating roller, thesheet separating performance is based on the function of the brakingtorque of the torque-limiter coupled onto the separating roller rotationaxis. Next, such the function of the torque-limiter will be described byusing a conventional example in which both of a pickup roller and a feedroller are used as shown in FIG. 23.

First, when a set torque of a torque-limiter is given as T and a radiusof the separating roller 176 a is given as Rr, a tangential force Ta ona circumferential force necessary for rotating the separating roller 176a through the torque-limiter (hereinafter referred to as “rotationallowable tangential force”) can be expressed by the followingexpression (1). $\begin{matrix}{{Ta} = \frac{T}{Rr}} & (1)\end{matrix}$

Then, in a case where there is no sheet S between the separating roller176 a and the feed roller 175, and the feed roller 175 which is rotatingis directly brought into contact with the separating roller 176 a, aforce larger than the rotation allowable tangential force Ta is acted onthe separating roller 176 a. In other words, a large rotation torque isacted on the torque-limiter of the separating roller 176 a, thereby thetorque-limiter allows the separating roller 176 a to rotate inassociation with the feed roller 175.

In this case, when an urging force of the separating roller 176 aagainst the feed roller 175 is given as N, μg×N which expresses arotating force in a clockwise direction shown in FIG. 23 is added to theseparating roller 176 a by a coefficient of friction pg between theseparating roller 176 a and the feed roller 175. At this time, thefollowing expression (2) is established, thereby the separating roller176 a is driven by the feed roller 175 to rotate in a clockwisedirection.Ta<μg×N   (2)

Further, in a case where only one sheet S is conveyed between the feedroller 175 and the separating roller 176 a, a force larger than therotation allowable tangential force Ta is acted on the separating roller176 a. In other words, a large rotation torque is acted on thetorque-limiter of the separating roller 176 a, thereby thetorque-limiter allows the separating roller 176 a to rotate inassociation with the feed roller 175.

In this case, when a coefficient of friction between the separatingroller 176 a and the sheet S is given as μr, the following expression(3) is established, thereby the separating roller 176 a rotates in aclockwise direction to deliver the sheet S as shown in FIG. 23.Ta<μr×N   (3)

Further, in a case where the pickup roller 174 delivers two or moresheets S between the feed roller 175 and the separating roller 176 a, aforce smaller than the rotation allowable tangential force Ta is actedon the separating roller 176 a. In other words, a rotation torquesmaller than the set value is acted on the torque-limiter of theseparating roller 176 a, thereby the torque-limiter prevents theseparating roller 176 a from rotating in association with the feedroller 175.

In this case, when a coefficient of friction between the sheets S isgiven as μS, the following expression (4) is established, thereby theseparating roller 176 a stops rotating. In this case, the upper sheet Sin contact with the feed roller 175 is conveyed, and the lower sheet Sin contact with the separating roller 176 a is prevented from beingconveyed by the separating roller 176 a which is a halt state.

This is because surfaces of the feed roller 175 and the separatingroller 176 a are formed of a rubber material or the like having arelatively larger coefficient of friction and are usually constituted soas to satisfy μr>μS. As a result, a slippage is caused between the sheetS in contact with the feed roller 175 and the sheet S in contact withthe separating roller 176 a, to thereby separate the two sheets S fromeach other.Ta>μs×N   (4)

By the above operation of the torque-limiter, the sheets S are separatedand conveyed to a downstream side.

In FIG. 24, the rotation allowable tangential force Ta of the separatingroller 176 a is set to an abscissa and an urging force N with respect tothe feed roller of the separating roller 176 a is plotted to an ordinate(hereinafter, this graph is referred to as “relationship diagram betweenthe rotation allowable tangential force and the urging force”).Hereinafter, a coordinate on the relationship diagram between therotation allowable tangential force and the urging force is expressed by(Ta, N).

When the above-mentioned expressions (3) and (4) are solved with respectto N, the following inequalities (5) and (6) are obtained.$\begin{matrix}{N > \frac{Ta}{\mu\quad r}} & (5) \\{N < \frac{Ta}{\mu\quad s}} & (6)\end{matrix}$

Here, when [straight line L1: N=Ta/μr] obtained by the above inequality(5) is plotted onto the relationship diagram between the rotationallowable tangential force and the urging force, in a case where thecoordinate (Ta, N) is present at a right side with respect to thestraight line L1, a normal paper feeding operation cannot be performeddue to a slip between the feed roller 175 and the sheet S even when onlyone sheet S is placed between the feed roller 175 and the separatingroller 176 a.

Further, when [straight line L2: N=Ta/μs] obtained by the aboveinequality (6) is plotted onto the relationship diagram between therotation allowable tangential force and the urging force, in a casewhere the coordinate (Ta, N) is present at a left side with respect tothe straight line L2, a normal separating/feeding operation also cannotbe performed since the separating roller 176 a is rotated in associationwith the feed roller 175 and the two sheets S are not separated to beconveyed together.

As described above, in FIG. 24, a normal separating/feeding function canbe obtained by collaborating the feed roller 175 with the separatingroller 176 a within the range indicated by a cross-hatching portionwhich is from the left side with respect to the straight line L1 to theright side with respect to the straight line L2. It should be noted thatthe cross-hatching portion is referred to as a feeding area.

Accordingly, the normal separating/conveying operation can be performedby collaborating the feed roller 175 with the separating roller 176 a inthe following situations. That is, the rotation allowable tangentialforce Ta of the separating roller 176 a is set such that in a case wherethe radius Rr of the separating roller 176 a is fixed in the aboveexpression (1), for example, the set torque T of the torque-limiter isvaried, and the urging force N of the separating roller is set within arange from N1 to N2 in a case where the torque-limiter varying within arange from Ta1 to Ta2 shown in FIG. 24 is used.

Next, an operation of the urging force of the separating roller 176 awill be described.

As described above, with the structure in which the separating roller176 a is rotatably held at a rotating end portion of a supporting member176 c which is supported rotatably about a rotation axis 176 d, a forceincreasing in accordance with the torque of the torque-limiter isgenerated in addition to a spring force of the spring 176 e added by anurging force of the separating roller 176 a.

Next, the mechanism of the force will be described with reference toFIG. 25. In FIG. 25, when reference symbol F denotes a tangential forcewhich is received by the separating roller 176 a from the sheet S to beconveyed, reference symbol L denotes a length between a center of therotation axis 176 d of the supporting member 176 c and a center of theseparating roller 176 a (hereinafter referred to as “supporting memberlength”), reference symbol Sp denotes a spring force received by thespring 176 e, and reference symbol a denotes an angle made by thesupporting member 176 c with respect to a tangential direction betweenthe feed roller 175 and the separating roller 176 a (hereinafterreferred to as a “supporting member angle”).

In this case, by taking into consideration a balance of moment with therotation axis 176 d of the separating roller 176 a being as a center,the urging force N is solved, thereby the following expression (7) canbe obtained. $\begin{matrix}{{{{{Sp} \times L} + {F \times \left( {{Rr} + {L\quad\sin\quad\alpha}} \right)}} = {N \times L\quad\cos\quad\alpha}}{N = {\frac{{Sp} \times L}{L\quad\cos\quad\alpha} + \frac{F \times \left( {{Rr} + {L\quad\sin\quad\alpha}} \right)}{L\quad\cos\quad\alpha}}}} & (7)\end{matrix}$Here, in a case where F is smaller than Ta, the separating roller 176 adoes not rotate. At the time point when F is equal to Ta, the separatingroller 176 a starts rotating, and after the time point, F is keptconstant. Therefore, in a case where F is equal to Ta, a relationshipbetween the urging force N and the rotation allowable tangential forceTa when the separating roller 176 a is rotating can be expressed by thefollowing expression (8). $\begin{matrix}{{N = {\frac{{Sp} \times L}{L\quad\cos\quad\alpha} + \frac{{Ta} \times \left( {{Rr} + {L\quad\sin\quad\alpha}} \right)}{L\quad\cos\quad\alpha}}}{N = {{\left( \frac{{Rr} + {L\quad\sin\quad\alpha}}{L\quad\cos\quad\alpha} \right){Ta}} + \frac{Sp}{\cos\quad\alpha}}}} & (8)\end{matrix}$

At this time, when Rr, L, Sp, and θ are kept constant, the expression(8) can be expressed as the following expression (9). $\begin{matrix}{N = {{kTa} + {Y\left( {{Where},{k = {\frac{{Rr} + {L\quad\sin\quad\alpha}}{L\quad\cos\quad\alpha} = {{const}.}}},{Y = {\frac{Sp}{\cos\quad\alpha} = {{const}.}}}} \right)}}} & (9)\end{matrix}$

From the expression (9), it is apparent that when the separating roller176 a is rotating, the urging force N is linearly increased inaccordance with the rotation allowable tangential force Ta of theseparating roller 176 a, that is, the set torque value of thetorque-limiter of the separating roller 176 a.

FIG. 26 shows that the expression (9) is plotted onto the relationshipdiagram between the rotation allowable tangential force and the urgingforce shown in FIG. 24. It is apparent that the urging force N islinearly increased in accordance with the rotation allowable tangentialforce Ta of the separating roller 176 a, that is, the set torque valueof the torque-limiter of the separating roller 176 a.

It should be noted that a straight line L4 shown in FIG. 26, as shown inFIG. 27 for example, a separating roller 186 a is constituted so as tobe slidable in a direction toward a feed roller 185 and be urged by aspring 186 e. The urging force N in the structure in which the urgingforce N is determined only by the force of the spring 186 e of theseparating roller 186 a is plotted. It is known that, as compared withthis structure, in the structure in which the separating roller issupported by the supporting member which is rotatably supported, therotation allowable tangential force Ta having a range larger than thefeeding area indicated by a cross-hatching portion, that is, the torquevalue of the torque-limiter having a wider range can be set. Forexample, the technique is disclosed in JP 03272572 B.

The above-described operations of the torque-limiter and the urgingforce are similarly acted on the sheet feeding portion in which thepickup roller and the feed roller are provided separately.

However, the conventional sheet feeding apparatus, the image formingapparatus, and the image reading apparatus having a rotation axis with astructure in which a separating roller is supported by a supportingmember which is rotatably supported have a limitation to minimize thesize of the sheet separating portion.

Next, the reason for the above will be described. FIG. 28 is a diagramshowing a state in which, in the structure described in FIG. 23, thesheet S is conveyed by the feed roller 175 and the conveyed sheet Scollides against the separating roller 176 a. A direction of a conveyingforce of the conveyed sheet S colliding against the separating roller176 a is acted on a direction in which the separating roller 176 a isapart from the feed roller 175 in proportion to a relationship betweenthe supporting member 176 c and the rotation axis 176 d.

At this time, when the separating roller 176 a is made apart beingoverwhelmed by the conveying force of the sheet S, the separating roller176 a which is rotating driven by the feed roller 175 stops rotating.When the separating roller 176 a is thus stopped, the collided leadingend of the sheet S may be damaged and folded. Accordingly, with thestructure in which the separating roller 176 a is supported by thesupporting member 176 c which is rotatably supported, a spring forceenough to bear the conveying force of the sheet S must be required.

Here, in FIG. 28, reference symbol β denotes an angle (hereinafterreferred to as a “sheet feeding angle”) made by a line connecting acontact point between the feed roller 175 and the sheet S and a centerof the feed roller 175 and a normal line between the feed roller 175 andthe separating roller 176 a, reference symbol γ denotes an angle(hereinafter referred to as an “entry point angle”) made by a lineconnecting an entry point of the sheet S into the separating roller 176a and a center of the separating roller 176 a and a tangential line ofthe feed roller 175 and the separating roller 176 a, and referencesymbol P denotes a conveying force of the sheet S applied by the feedroller 175.

Here, the spring force for bearing the conveying force P of the sheet Smust be satisfied even when the tangential force received by theseparating roller 176 a from a rotation of the feed roller 175 isexcluded. Next, the conditional expression obtained from the balance ofrotation moment at the rotation axis 176 d of the supporting member 176c, is expressed as follows.Sp×L+P cos β×(L sin α+Rr sin γ)−N×L cos α+P sin β×(L cos α+Rr cos γ)In this expression, for the spring pressure of the separating rollerenough to bear the conveying force of the sheet S, the spring pressureof the separating roller which satisfies N>0 is an essential condition.Then, the condition of N>0 is added to the above expression, a conditionfor the spring force for bearing the conveying force of the sheet isgiven by the following expression (10). $\begin{matrix}{{Sp} > {\frac{P}{L}\left\{ {{L \times {\sin\left( {\beta - \alpha} \right)}} + {{Rr} \times {\sin\left( {\beta - \gamma} \right)}}} \right\}}} & (10)\end{matrix}$For example, FIG. 29 shows arrangements by four combinations (A to D) ofthe length and the angle between the supporting member 176 c and therotation axis 176 d. With respect to the four combinations (A to D), thespring pressure of the separating roller for bearing the conveying forceof the sheet is calculated, and the result is plotted on therelationship diagram between the rotation allowable tangential force andthe urging force shown in FIG. 24.

Here, when the radius Rr of the separating roller 176 a is set to 20 mm,the sheet feeding angle β is set to 38°, and the entry point angle γ isset to 57°. In obtaining the conveying force P of the sheet to beentered, when the pressure of the sheet stacking plate pressurizingspring 152 b is set to 0.4 N and the coefficient of friction between thefeed roller 175 and the sheet S is set to 2 (in this case, the pressureof the sheet stacking plate pressurizing spring 152 b and thecoefficient of friction are need to be calculated on the maximum value),the conveying force P of the sheet is 0.8 N, which is used to calculatethe conveying force P of the sheet to be entered.

In addition, reference symbols A to D indicate various combinations ofthe separating roller supporting angle α and the separating rollersupporting member length L. The following Table 1 shows a relationshipbetween a and L, the spring pressure Sp of the separating roller forbearing the sheet conveying force P obtained from the above expression(10), and the proportional multiplier k with respect to the rotationallowable tangential force Ta obtained from the above expression (8).TABLE 1 α(Deg) L(mm) Sp(N) k A 0 20 0.35 0.5 B 15 20 0.18 0.79 C 15 300.22 0.61 D 30 30 0 0.96

It is apparent that when the separating roller supporting angle α isincreased in the relationship between A and B shown in Table 1, thespring pressure Sp of the separating roller for bearing the sheetconveying force P is decreased and the proportional multiplier k isincreased. It is also apparent that when the separating rollersupporting member length L is increased in the relationship between Band C shown in Table 1, the spring pressure Sp of the separating rollerfor bearing the sheet conveying force P is increased and theproportional multiplier k is decreased. In the position D, since therotation axis 176 d of the supporting member 176 c is positioned at thelower part of FIG. 29 with respect to the direction of the conveyingforce P of the sheet S, the conveying force P of the sheet S is notworked in the direction in which the separating roller 176 a is madeapart from the feed roller 175, and the conveying force P of the sheet Sis worked in the direction in which the separating roller 176 a isjammed into the feed roller 175. Therefore, in the position D, thespring pressure Sp of the separating roller can be arbitrarily setwithout taking into consideration the effect of the sheet conveyingforce P.

FIG. 30 shows a result that the result of Table 1 is substituted for theabove expression (8) and the result is plotted on the relationshipdiagram between the rotation allowable tangential force and the urgingforce. In this case, a coefficient of friction μS between the sheets Sand a coefficient of friction μr between the sheet S and the separatingroller 176 a are set to the value approximate to the value described inJP 03272572 B, a coefficient of friction μS between the sheets S is setto 0.8, and a coefficient of friction μr between the sheet S and theseparating roller 176 a is set to 1.0. The rotation allowable tangentialforce Ta is set within a range from 0.2 N to 0.4 N.

Then, as shown in FIG. 30, in the case of A, the value of the springpressure Sp of the separating roller is too high to intersect with thefeeding area. In the case of B, though the value of the spring pressureSp of the separating roller is small, the proportional multiplier k isso large that an overlapping area sufficient for the feeding area cannotbe obtained. In the case of C, though it is better as compared with thecases of A and B, an overlapping area sufficient for the feeding areacannot be obtained.

On the other hand, in the case of D, since the sheet conveying force Pdoes not affect the spring pressure Sp of the separating roller, thespring pressure Sp of the separating roller can be arbitrarily set. Withsuch the advantage, the spring pressure of the separating roller can beset in accordance with the desired rotation allowable tangential forceTa as shown in FIG. 30.

However, even in the case D, it is necessary that the separating rollersupporting angle α is increased and the supporting member length L isincreased in order to secure the overlapping area sufficient for thefeeding area. In other words, it is necessary that the supporting member176 c is inclined in a direction in which the sheet width is increasedand the length of the supporting member 176 c is increased in adirection of the sheet width.

As described above, in the conventional rotation supporting method ofsupporting the separating roller by the supporting member which isrotatably supported, the supporting member having at least a width abouttwice or more of the width of the separating roller is required. As aresult, there is a limitation to minimize the size of the sheetseparating portion. Further, since the separating roller and the sheetseparating portion constituted of the separating pad described abovehave substantially the same width, it is difficult to compactly arrangethe sheet separating portion as in the sheet feeding portion mounted onthe printer described with reference to FIGS. 19 and 20 by adopting therotation supporting method of supporting the separating roller by thesupporting member which is rotatably supported.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems, andtherefore has an object to provide a sheet feeding portion (sheetfeeding apparatus), an image forming apparatus, and an image readingapparatus which are capable of securing the sheet feeding performanceand minimizing the space for the sheet feeding portion.

According to one aspect of the present invention,

a sheet stacking member configure to support sheets;

a sheet feeding member configure to feed the sheet from said sheetstacking member;

a sheet separating portion including a separating roller provided so asto abut against said sheet feeding member and a torque-limiter forallowing said separating roller to rotate in association with said sheetfeeding member when a rotation torque larger than a predetermined torqueis acted on said separating roller and for preventing said separatingroller from rotating in association with said sheet feeding member whenthe rotation torque equal to or smaller than the predetermined torque isacted on said separating roller;

a guide member configure to support said sheet separating portion in aslidable manner and guiding said sheet separating portion with apredetermined angle in a direction different from a direction alongwhich said sheet feeding member is opposed to said separating roller;and

an urging member configure to urge said sheet separating portion so thatsaid separating roller is in pressure contact with said sheet feedingmember.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a structure of an electrophotographicprinter as an example of an image forming apparatus including a sheetfeeding apparatus according to a first embodiment of the presentinvention;

FIG. 2 is a diagram for explaining a structure of the sheet feedingapparatus;

FIG. 3 is a diagram for explaining a method of connecting a separatingroller to a torque-limiter in a sheet separating portion provided to thesheet feeding apparatus;

FIG. 4A is a diagram for explaining another method of connecting aseparating roller to a torque-limiter in a sheet separating portion, andFIG. 4B is a diagram for explaining another pressurizing method for aseparating roller spring in the sheet separating portion;

FIG. 5 is a diagram for explaining a function of an urging force withrespect to a feed roller of the separating roller in the sheetseparating portion;

FIG. 6 is a diagram for explaining a state of a force working when theseparating roller in the sheet separating portion collides with a sheet;

FIG. 7 is a relationship diagram between the rotation allowabletangential force and the urging force in the sheet separating portion;

FIGS. 8A and 8B are diagrams for explaining a method of changing anangle made by a guide surface of a guide member in the sheet separatingportion.

FIG. 9 is a schematic diagram of a structure of a printer according toan application example of the first embodiment;

FIG. 10 is a diagram for explaining a structure of a sheet feedingapparatus provided to the printer;

FIG. 11 is a diagram for explaining a structure of a drawer of a sheetfeed cassette of the printer;

FIG. 12 is a perspective view of the sheet separating portion of thesheet feeding apparatus provided to the printer;

FIG. 13 is a diagram for explaining a structure of a sheet feedingapparatus according to a second embodiment of the present invention;

FIG. 14 is a schematic diagram for showing a structure of anelectrophotographic printer as an example of an image forming apparatusincluding a sheet feeding apparatus according to a third embodiment ofthe present invention;

FIG. 15 is a diagram for explaining a structure of a multiple sheetfeeding portion provided to the printer;

FIGS. 16A and 16B are diagrams for explaining a structure of a sheetfeeding apparatus according to a fourth embodiment of the presentinvention;

FIGS. 17A and 17B are diagrams for explaining another structure of thesheet separating portion provided to the sheet feeding apparatus;

FIGS. 18A and 18B are diagrams for explaining another structure of thesheet separating portion provided to the sheet feeding apparatus;

FIG. 19 is a schematic diagram for showing a structure of a conventionalprinter;

FIG. 20 is a diagram for explaining a structure of the sheet feedingportion provided to the conventional printer;

FIG. 21 is a schematic diagram for showing a structure of anotherconventional printer;

FIG. 22 is a diagram for explaining a structure of the sheet feedingportion provided to the conventional printer;

FIG. 23 is a diagram for explaining another structure of a conventionalsheet feeding portion;

FIG. 24 is a relationship diagram between the rotation allowabletangential force and the urging force in a conventional sheet separatingportion;

FIG. 25 is a diagram for explaining a function of a conventional urgingforce N;

FIG. 26 is a graph in which the urging force N is plotted on therelationship diagram between the rotation allowable tangential force andthe urging force in the conventional sheet separating portion;

FIG. 27 is a diagram for explaining an urging method for a conventionalseparating roller;

FIG. 28 is a diagram for explaining a state in which the sheet collideswith the separating roller;

FIG. 29 is a diagram for explaining a state in which the angle and thelength of a conventional supporting member are modified; and

FIG. 30 is a graph in which the structure shown in FIG. 29 is plotted onthe relationship diagram between the rotation allowable tangential forceand the urging force in the conventional sheet separating portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the best modes for carrying out the present invention willbe described with reference to the drawings.

FIG. 1 is a schematic diagram of a structure of an electrophotographicprinter as an example of an image forming apparatus including a sheetfeeding apparatus according to a first embodiment of the presentinvention. However, dimensions, materials, configurations, relativearrangement, and the like of the structural elements described in thisembodiment do not limit the present invention thereto as long as thereis no particular specific description.

In FIG. 1, reference numeral 1 denotes a printer main body (hereinafterreferred to as an “apparatus main body”) and reference numeral 1Adenotes an image forming portion. Below the image forming portion 1A, asheet feeding apparatus 3 is arranged. The image forming portion 1Aincludes a laser scanner 10, an image forming process unit 9 includingan image forming drum 9 a serving as an image bearing member, and atransfer roller 9 b for transferring a toner image formed on thephotosensitive drum 9 a onto a sheet S.

Reference numeral 11 denotes a fixing device for fixing the toner imageon the sheet after the toner image formed by the image forming portion1A is transferred onto the sheet. After the toner image is fixed on thesheet by the fixing device 11, the sheet S is sequentially discharged toand stacked on a discharge tray 14 provided on the uppermost part of themain body of an apparatus.

Further, as shown in FIG. 2, a sheet feeding apparatus 3 includes asheet feed cassette 2 serving as a sheet stacking member, a feed roller5 serving as a sheet feeding member for feeding the sheets stored in thesheet feed cassette 2, a separating roller 6 a brought into pressurecontact with the feed roller 5, and a sheet separating portion 6 forseparating the sheets S fed by the feed roller 5 one by one.

Here, the feed roller 5 also serves as a pickup roller and is rotatablyprovided to the sheet feed cassette 2. The feed roller 5 is in contactwith the uppermost sheet S1 stacked on a sheet stacking plate 2 a forstacking the sheets S, and at the same time, the feed roller 5 is incontact with the separating roller 6 a at a downstream side in a sheetconveying direction. It should be noted that the sheet stacking plate 2a is pressed upwardly by a sheet stacking plate pressure spring 2 b fromthe back surface side (lower side in FIG. 1) of the sheet stacking plate2 a, and a leading end of the uppermost sheet S1 stacked on the sheetstacking plate 2 a is pressed against the feed roller 5.

Then, in the printer provided with the sheet feeding apparatus 3 havingthe above structure, when the feed roller 5 rotates counterclockwise inFIGS. 1 and 2 owing to a drive of a driving motor (not shown), theuppermost sheet S1 stacked on the sheet stacking plate 2 a is received,and then the sheet S1 is separated from other sheets by the sheetseparating portion 6 to be conveyed in a downstream direction.

Thus, the separated sheet S1 is then conveyed to the transfer portionconstituted of the electrophotographic photosensitive drum 9 a and thetransfer roller 9 b through a pair of conveying rollers 7 and a pair ofregistration rollers 8. At this time, on a surface of theelectrophotographic photosensitive drum 9 a, a toner image is formed bya laser beam outputted from the laser scanner 10 positioned above theelectrophotographic photosensitive drum 9 a. The toner image is thentransferred onto the conveyed sheet S1 at the transfer portion.

Then, the sheet S1 onto which the toner image has been transferred isconveyed in a downstream direction, and the toner image is fused andfixed on the sheet S1 after being heated and pressurized by the fixingdevice 11. After that, thus processed sheet S is sequentially stacked onthe discharge tray 14 through a pair of conveying rollers 12 and a pairof discharge rollers 13 which serve as sheet discharge means.

On the other hand, the sheet separating portion 6 of the sheet feedingapparatus 3 includes, as shown in FIG. 2, the separating roller 6 a, asupporting member 6 c, a separating roller spring 6 d serving as anurging member, and a guide member 6 e. The separating roller 6 a ispositioned at a downstream side of a contact point between the feedroller 5 and the uppermost sheet S1. In addition, the guide member 6 eis fixed to the apparatus main body 1 and for slidably supporting theseparating roller 6 a, the supporting member 6 c, and the separatingroller spring 6 d. The separating roller 6 a is slidable in a verticaldirection while being guided to a flat guide surface of the guide member6 e.

Here, since the separating roller 6 a is rotatably supported at a topend of the supporting member 6 c and is urged on the upper side by theseparating roller spring 6 d through the separating member 6 c, theseparating roller 6 a is contacted with the feed roller 5 slidably in avertical direction.

In other words, in this embodiment, the separating roller 6 a is notslidably contacted by pressure in a direction in which the feed roller 5is opposed to the separating roller 6 a, that is, in a direction inwhich the separating roller 6 a surfaces the center of the feed roller5, but is slidably constituted by the guide member 6 e in a verticaldirection. Thus, the separating roller 6 a is pressure-contacted withthe feed roller 5 by a predetermined angle with respect to a directionin which the separating roller 6 a surfaces the center of the feedroller 5. Then, by constituting the separating roller 6 a which isslidable in a vertical direction as described above, the sheetseparating portion 6 can be constituted with an area substantially equalto the width (diameter) of the separating roller 6 a.

On the other hand, the separating roller 6 a is not driven by a motor orthe like and is connected to a torque-limiter 6 b on the rotation axis.FIG. 3 is a diagram for explaining a method of connecting the separatingroller 6 a to the torque-limiter 6 b which is viewed from a sheetconveying direction of the sheet separating portion 6.

Here, the separating roller 6 a is rotatably connected to a rotationaxis 6 f through the torque-limiter 6 b. The separating roller 6 a isrotatable when rotation torque equal to or larger than the set torque ofthe torque-limiter 6 b is generated. The method of connecting thetorque-limiter 6 b is not limited to this, and it is possible to connectthe torque-limiter 6 b so as to be substantially embedded in theseparating roller 6 a as shown in FIG. 4A.

Further, a pressurizing method for the separating roller spring 6 d isnot also limited to the method shown in FIG. 3, it is possible to adopta pressurizing method for two separating roller springs 6 d, forexample, as shown in FIG. 4B, as long as elasticity working in adirection in which the feed roller 5 is pressurized along the guidemember 6 e is obtained.

The sheet separating performance of the sheet separating portion 6 isbased on the function of the braking torque of the torque-limiter 6 b inconnection with the separating roller rotation axis.

Here, in relation to the function and sheet separating performance ofthe torque-limiter 6 b, the above-described relationships (1) to (6) areestablished. In the relationship diagram between the rotation allowabletangential force and the urging force shown in FIG. 24, the feeding areais established. Accordingly, in a case where one sheet is placed betweenthe feed roller 5 and the separating roller 6 a, the rotation allowabletangential force Ta and the urging force N are required to be controlledto be within the feeding area of FIG. 24 so that the torque-limiter 6 ballows the separating roller 6 a to rotate in association with the feedroller 5 without a slip. When two sheets are placed between the feedroller 5 and the separating roller 6 a, the torque-limiter 6 b preventsthe separating roller 6 a from rotating in association with the feedroller 5 to separate the sheets.

Next, a function of the urging force N with respect to the feed roller 5of the separating roller 6 a according to this embodiment will bedescribed. Also in this embodiment, as described above, the urging forceN is increased in accordance with the rotation allowable tangentialforce Ta of the separating roller 6 a, that is, the set torque value ofthe torque-limiter 6 b of the separating roller 6 a when the separatingroller 6 a is rotating.

Such the mechanism will be described with reference to FIG. 5. FIG. 5 isa diagram for explaining a state in which one sheet S is conveyed whilebeing sandwiched between the feed roller 5 and the separating roller 6a. It should be noted that in FIG. 5, the forces working similarly tothose of FIG. 25 are indicated by the identical reference symbols andthe explanation thereof is omitted. In addition, reference symbol “f”denotes a reaction force received from the guide surface of the guidemember 6 e, reference symbol “n” denotes a friction force between theguide surface of the guide member 6 e and the supporting member 6 c, andreference symbol θ denotes an angle made by the guide surface of theguide member 6 e which is a predetermined angle with respect to adirection in which the feed roller 5 is opposed to the separating roller6 a (hereinafter referred to as a “guide surface angle”).

In FIG. 5, when a horizontal direction is set to an X direction and avertical direction is set to a Y direction, the following expression canbe obtained with respect to a balance of two directions with theseparating roller 6 a as a center.

Expression representing a force balance in the X direction:N×sin θ+F×cos θ=f   (11)

Expression representing a force balance in the Y direction:N×cos θ+n=F×sin θ+Sp   (12)Here, in a case where F is smaller than Ta, the separating roller 6 adoes not rotate. At the time when F is equal to Ta, the separatingroller 6 a starts rotating, and after that, F is kept constant and N isalso made constant. In addition, when a coefficient of friction betweenthe guide member 5 e and the supporting member 6 c is μn, n=μn×f isestablished.

The above conditional expression is substituted for the expressions (11)and (12), and such the expression is solved with respect to N, arelational expression between the urging force N when the separatingroller 6 a is rotating and the rotation allowable tangential force Ta ofthe separating roller 6 a is expressed as the following expression (13).$\begin{matrix}{N = {{\frac{{\sin\quad\theta} - {\mu\quad n \times \cos\quad\theta}}{{\cos\quad\theta} + {\mu\quad n \times \sin\quad\theta}}{Ta}} + {\frac{1}{{\cos\quad\theta} + {\mu\quad n \times \sin\quad\theta}}{Sp}}}} & (13)\end{matrix}$Then, as apparent from the expression (13), when a position of theseparating roller 6 a is fixed, in other words, when the value θ isfixed, $\begin{matrix}{N = {{kTa} + {Y\left( {{Where},{k = {\frac{{\sin\quad\theta} + {\mu\quad n \times \cos\quad\theta}}{{\cos\quad\theta} - {\mu\quad n \times \sin\quad\theta}} = {const}}},{Y = {{\frac{1}{{\cos\quad\theta} - {\mu\quad n \times \sin\quad\theta}}P} = {{const}.}}}} \right)}}} & (14)\end{matrix}$is obtained. When the separating roller 6 a is rotating, the urgingforce N is linearly increased in accordance with the rotation allowabletangential force Ta of the separating roller 6 a, that is, the settorque value of the torque-limiter of the separating roller 6 a.

Further, assuming that the value μn is sufficiently small, theexpression (13) can be expressed by the expression (15). $\begin{matrix}{{N = {{\frac{\sin\quad\theta}{\cos\quad\theta}{Ta}} + \frac{Sp}{\cos\quad\theta}}}{N = {{\left( {\tan\quad\theta} \right){Ta}} + \frac{Sp}{\cos\quad\theta}}}} & (15)\end{matrix}$

Next, a function in a case where the sheet S fed from the feed roller 5collides with the separating roller 6 a with reference to FIG. 6. FIG. 6is a diagram for explaining a state of a force working when theseparating roller 6 a according to this embodiment collides with thesheet S.

Here, when the sheet S collides with the separating roller 6 a, theseparating roller 6 a is required to be contact with the feed roller 5without being apart from the feed roller 5 by being overwhelmed by theconveying force of the sheet S even when the tangential force receivedfrom the feed roller 5 is excluded.

Then, as a condition for the separating roller 6 a not to be apart fromthe feed roller 5 but to be contact with the feed roller 5, thefollowing expression can be obtained with respect to a balance of twodirections with the separating roller 6 a as a center in a case where anangle between a conveying direction of the sheet S and the guide member6 e is set to δ and a horizontal direction and a vertical direction areset to X and Y directions, respectively, as shown in FIG. 6.

Expression representing a force balance in the X direction:$\begin{matrix}{{{P\quad{\cos\left( {\frac{\pi}{2} - \delta} \right)}} + {N\quad\sin\quad\theta}} = f} & (16)\end{matrix}$

Expression representing a force balance in the Y direction:$\begin{matrix}{{{P\quad{\sin\left( {\frac{\pi}{2} - \delta} \right)}} + {Sp}} = {{N\quad\cos\quad\theta} + n}} & (17)\end{matrix}$

When the conditional expression n=μn×f is substituted for theexpressions (16) and (17), and when the substituted expression is solvedwith respect to N, the following expression is obtained. $\begin{matrix}{N = \frac{{P\quad\cos\quad\delta} - {\mu\quad n\quad P\quad\sin\quad\delta} + {Sp}}{{\cos\quad\theta} + {\mu\quad n\quad\sin\quad 0}}} & (18)\end{matrix}$In the expression (18), with respect to the spring pressure of theseparating roller enough to bear the conveying force of the sheet S, thespring pressure of the separating roller of N>0 is a necessarycondition, so when the above expression (18) is added with the conditionof N>0, a spring force enough to bear the conveying force of the sheetis obtained by the following expression (19).Sp>P(μn P sin δ−cos δ)   (19)At this time, assuming that the value μn is sufficiently small, theexpression (19) is expressed as the following expression (20).Sp>P(−cos δ)   (20)Then, when δ is smaller than 90° in this expression (20), P (−cos δ)<0is established, and it is apparent from the expression that the springpressure of the separating roller can be arbitrarily set regardless ofthe conveying force of the sheet S. To the contrary, when δ is largerthan 90° in this expression (20), P (−cos δ)>0 is established, and it isapparent from the expression that the spring force is affected by theconveying force of the sheet S and the spring force for bearing theconveying force of the sheet S must be set.

This is because when δ is smaller than 90°, a component of force of thesheet conveying force P is received by the reaction force “f” from theguide member 6 e and the reaction force of the feed roller 5 (urgingforce N), so the force is acted on the separating roller 6 a in anupward direction in FIG. 6 and is not worked in a downward direction inFIG.6. In other words, since the separating roller 5 a is not made apartfrom the feed roller 4 by the collision of the sheet S, the leading endof the sheet S is prevented from being damaged by stopping rotation ofthe separating roller 5 a as described above.

However, when 5 is larger than 90°, a component of force of the sheetconveying force P is received by the reaction force “f” from the guidemember 6 e and the force Sp of the separating roller spring 6 d, so theforce is acted on the separating roller 6 a in a downward direction.Accordingly, a spring for not being overwhelmed by the conveying forceof the sheet S must be set on the basis of the above expression.

As described above, when the angle 6 between the conveying direction ofthe sheet S and the guide member 6 e is set to be equal to or smallerthan 90°, the sheet conveying force P can be prevented from working in adirection in which the separating roller 6 a is made apart from the feedroller 5. As a result, since the spring pressure of the separatingroller spring 6 d of the separating roller is not limited by thecollision of the sheet S, the spring pressure of the separating rollercan be arbitrarily set in order to stabilize the separating/conveyingoperation to be described later.

Next, an influence on the sheet feeding performance will be verifiedwhen the structure according to this embodiment is plotted on therelationship diagram between the rotation allowable tangential force andthe urging force.

FIG. 7 is the relationship diagram between the rotation allowabletangential force and the urging force in the sheet separating portion 6according to this embodiment. As described above, when the separatingroller 6 a and the guide member 6 e are arranged so that the angle δbetween the conveying direction of the sheet S and the guide member 6 eis set to be equal to or smaller than 90°, the urging force N(Y-intercept of FIG. 7) when Ta is equal to 0 can be arbitrarily setsince the sheet S is not limited by the collision with the separatingroller 6 a. In addition, a slope can be changed in accordance with theangle made by the guide member 6 e, and the guide surface angle θ can beselected in accordance with the desired rotation allowable tangentialforce Ta, that is, the torque value of the torque-limiter.

For example, it is assumed that a straight line A passing through thepoints (200, 250) and (400, 400) is set with the separating roller 6 a,the guide member 6 e, and the separating roller spring Sp. From theabove expression (15), the slope of the straight line A is obtained bytan θ. Thus, the guide surface angle θ is expressed as the followingexpression.${\tan^{- 1}\theta} = {\frac{400 - 250}{400 - 200} = {\frac{150}{200} = 0.75}}$θ = 36.8  (Deg)

Further, from the expression (15), the value of Y-intercept of FIG. 7 isexpressed as Sp/cos θ. Thus, Sp=0.8N can be set regardless of theinfluence of the sheet conveying force P.

A straight line B shown in FIG. 7 passes through the point (200, 250)and the slope of the strait line B is 1. In a case where the straightline B is set with the separating roller 6 a, the guide member 6 e, andthe spring pressure Sp of the separating roller, the guide surface angleθ and the spring pressure Sp of the separating roller may be set asfollows.tan⁻¹θ=1θ=45(deg.)Further, from the above expression (15), since the value of Y-interceptof FIG. 7 is expressed as Sp/cos θ, Sp=0.34 N is established.

As described above, the guide surface angle θ and the spring pressure Spof the separating roller can be arbitrarily set as described above. Itshould be noted that the upper limit of the slope of FIG. 7 is expressedas the following expression (21) so as to generate an intersection withthe straight line of N=1/μS×Ta. $\begin{matrix}{{\tan\quad\theta} < \frac{1}{\mu\quad s}} & (21)\end{matrix}$

Accordingly, the condition for θ is expressed as the followingexpression (22). $\begin{matrix}{\theta < {\tan^{- 1}\left( \frac{1}{\mu\quad s} \right)}} & (22)\end{matrix}$

For example, when μS is equal to 0.8, the upper limit of θ is 51.3°.

As described above, when the guide member 6 e is arranged so that theangle δ between the conveying direction of the sheet S and the guidemember 6 e is set to be equal to or smaller than 90°, the sheetconveying force P can be prevented from working in a direction in whichthe separating roller 6 a is made apart from the feed roller 5. As aresult, since the spring pressure Sp of the separating roller is notlimited by the collision of the sheet S, the spring pressure of theseparating roller can be arbitrarily set.

Further, the guide surface angle θ can be set with the upper limit asthe above expression (22) in accordance with the desired rotationallowable tangential force Ta, that is, the desired torque-limiter valueof the separating roller 6 a.

As a result, in the sheet separating portion 6 constituted within asmall space having a width substantially equal to that of the separatingroller as described above, the sheet feeding performance of preventingdouble feeding and slip can be secured, and thus the space for the sheetseparating portion 6 and the sheet feeding apparatus 3 can be minimized.

Therefore, the sheet separating portion 6, which can give the sheetfeeding performance more stable than the sheet separating means usingthe separating pad, can be constituted by a width equal to that of thesheet separating means using the separating pad. In addition, the sheetseparating portion 6 can be constituted by a half width as compared withthe rotation supporting method of supporting the separating roller by asupporting member which is supported by a conventional rotation axis asshown in FIG. 21 and the like as described above.

As a method of changing the guide surface angle θ, there are twomethods: a method of changing the guide surface angle θ by rotating theseparating roller 6 a and the guide member 6 e with the feed roller 5 asthe center as shown in FIG. 8A; and a method of changing the guidesurface angle θ by rotating the guide member 6 e with the separatingroller 6 a as the center as shown in FIG. 8B. In both of the methods,when δ is smaller than 90° and the condition of the expression (22) issatisfied, the above effect can-be obtained.

As described above, when the sheet separating roller 6 a is slidablysupported by the guide member 6 e and is guided with a predeterminedangle in a direction different from a direction in which the feed roller5 is opposed to the separating roller 6 a, the sheet separating portion6 can be constituted by a width substantially equal to that of theseparating roller. With this structure, the space for the sheetseparating portion 6 and the sheet feeding apparatus 3 can be minimizedwhile securing the feed performance.

Next, as an application example of this embodiment, a printer includinga multiple-stage sheet feeding apparatus and an option sheet feedingapparatus will be described.

FIG. 9 is a schematic diagram of a structure of a printer according toan application example of this embodiment. In FIG. 9, a referencenumeral 51 is a printer main body (hereinafter referred to as an“apparatus main body”), reference numeral 51A is an image formingportion. Below the image forming portion 51A, an upper sheet feedingapparatus 52 and a lower sheet feeding apparatus 56 are arranged byoverlapping with each other. The image forming portion 51A is providedwith a laser scanner 64, an image forming process unit 63 including aphotosensitive drum 63 a as an image bearing member, a transfer roller63 b for transferring a toner image formed on the photosensitive drum 63a onto a sheet S, and the like.

Reference numeral 65 denotes a fixing device for fixing a toner image ona sheet after the toner image formed by the image forming portion 51A istransferred onto the sheet. After the toner image is fixed by the fixingdevice 65, the sheet S is sequentially discharged to and stacked on adischarge tray 69 provided on an uppermost portion of the apparatus mainbody. In addition, reference numeral 81 denotes an option sheet feedingapparatus provided with a sheet feeding apparatus 82, and the optionsheet feeding apparatus 81 is selectively mounted on the bottom surfaceof the apparatus main body 51.

In FIG. 10, reference numeral 70 denotes a conveying path for the optionsheet feeding apparatus for conveying the sheet S fed from the optionsheet feeding apparatus 81 upwardly (to a downstream side), theconveying path 70 for the option sheet feeding apparatus merges into aconveying path 71 for the lower sheet feeding portion for conveying thesheet S fed from the lower sheet feeding apparatus 56 upwardly (to thedownstream side).

Further, a conveying path 72 for the upper sheet feeding portion forconveying the sheet S fed from the upper sheet feeding apparatus 52upwardly (to the downstream side) merges into the conveying path 71 forthe lower sheet feeding portion. Thus, the sheets fed from the optionsheet feeding apparatus 81, the lower sheet feeding apparatus 56, andthe upper sheet feeding apparatus 52 pass through a sheet feedingportion merging conveying path 73 to be conveyed to the downstream side.

Then, the sheet S which has passed through the sheet feeding portionmerging conveying path 73 to be conveyed to the downstream side isconveyed to the transferring portion constituted of the photosensitivedrum 63 a and a transfer roller 63 b, in synchronism with an imageformed on the photosensitive drum 63 a by a registration roller 62 shownin FIG. 9. In the transferring portion, a toner image is transferredonto the sheet S.

It should be noted that the sheet S onto which the toner image has beentransferred is then conveyed to the fixing device 65, and the tonerimage is fixed on the sheet S by applying pressure and heat at thefixing device 65. After that, the sheet S is conveyed through conveyingroller pairs 66 and 67 and a discharge roller pair 68, and issequentially discharged to and stacked on the discharge tray 69.

Next, the upper sheet feeding apparatus 52 and an upper sheet feedcassette 55 provided to the upper sheet feeding apparatus 52 will bedescribed. As shown in FIG. 10, similar structures apply to the sheetfeeding apparatus 82 provided with a feed roller 83 and the sheetseparating portion 84 which are provided to the option sheet feedingapparatus 81, the lower sheet feeding apparatus 56 provided with thefeed roller 57 and the sheet separating portion 58, and sheet feedcassettes 59 and 85 provided to the option sheet feeding apparatus 81and the lower sheet feeding apparatus 56, respectively.

The upper feeding cassette 55 is provided with a sheet stacking plate 55a for stacking the sheets S in a rotatable manner as shown in FIG. 10.The sheet stacking plate 55 a is pressed upwardly by a sheet stackingplate pressure spring 55 b serving as the urging member, from a backsurface side (lower side in FIG. 10) of the sheet stacking plate 55 a. Aleading end of the uppermost sheet S1 stacked on the sheet stackingplate 55 a is pressed against the upper feed roller 53.

Then, the upper sheet feed roller 53 is contacted with the upper sheetseparating portion 54 at a downstream side of the sheet conveyingdirection. It should be noted that functions of a separating roller 54a, a torque-limiter 54 b, a supporting member 54 c, a separating rollerspring 54 d, and a guide member 54 e which constitute the upper sheetseparating portion 54, a positional relationship between each of thosecomponents and the upper feed roller 53, and an angle made when thesheet S collides with the separating roller 54 a are similar to those ofthe above embodiment. Accordingly, since the effects and the functionthereof are similar to those of the embodiment, an explanation thereofwill be omitted.

Further, the upper sheet feed cassette 55 is structured so as to bedrawn out in a width direction orthogonal to the sheet conveyingdirection as shown in FIG. 11. The upper sheet feed cassette 55 is drawnout along rails 75 a and 75 b which are positioned at both ends of theupper sheet feed cassette 55, as shown in FIGS. 9, 10, and 11.

In FIG. 11, reference symbol 55 c denotes an upper sheet-side regulatingplate for regulating a position in a width direction of a sheet bundleprepared on the sheet stacking plate 55 a, and reference symbol 55 ddenotes an upper sheet-back-end regulating plate for regulating aback-end position of a sheet bundle prepared on the sheet stacking plate55 a. The upper sheet separating portion 54 is mounted at a side of thesheet conveying direction of the upper sheet feed cassette 55. In otherwords, in this application example, the upper feed roller 53 remains inthe apparatus main body 51, the upper sheet separating portion 54 isdetachably attached to the apparatus main body 51 together with uppersheet feed cassette 55 as shown in FIG. 12.

In this application example, as shown in FIG. 10, a bottom surface ofthe upper sheet feed cassette 55 is overlapped with the lower feedroller 57 of the lower sheet feeding apparatus 56, to thereby reduce theheight of the apparatus main body by the overlapped portion. Since theupper sheet separating portion 54 requires only a small space, the uppersheet separating portion 54 can be close to a position in a lateraldirection of the lower feed roller 57. As a result, a compact productcan be realized in which the width and height of the apparatus main bodycan be suppressed.

In addition, when the option sheet feeding apparatus 81 is added and theconveying path 70 for the option sheet feeding apparatus is arranged tobe adjacent to the lower sheet separating portion 58, the lower sheetseparating portion 58 requires only a small space. Thus, a compactproduct can be realized in which the width of the apparatus main bodycan be suppressed.

In the above description, though the feed roller which can be used as apickup roller is used, the present invention is not limited to this. Itis possible to adopt a sheet feeding apparatus using a feed roller and apickup roller separately.

Next, a second embodiment of the present invention will be described inwhich a feed roller and a pickup roller are separately used.

FIG. 13 is a diagram for explaining a sheet feeding apparatus accordingto the second embodiment of the present invention. It should be notedthat in FIG. 13, the same reference symbols as those of FIG. 2 indicatethe identical or corresponding portions, and explanations thereof areomitted.

In FIG. 13, reference numeral 4 denotes the pickup roller and referencenumeral 5A denotes a feed roller. The feed roller 5A is positioned at adownstream side of the pickup roller 4. The separating roller 6 aprovided to the sheet separating portion 6 opposite to the feed roller5A is contacted with the feed roller 5A. The separating roller 6 a issupported by the guide member 6 e with respect to the feed roller 5Awith a predetermined angle in a slidable manner. In addition, the sheetstacking plate 2 a is urged by the sheet stacking plate pressure spring2 b such that a leading end of the uppermost sheet S1 stacked on thesheet stacking plate 2 a is pressed against the pickup roller 4

Then, when the pickup roller 4 is rotated counterclockwise as shown inFIG. 13 by a drive of a driving motor (not shown), a conveying force isapplied to the uppermost sheet S1 stacked on the sheet stacking plate 2a, and then the sheet S1 is conveyed between the feed roller 5A and theseparating roller 6 a. After that, the sheets S are separated one by oneto be conveyed to the image forming portion or the image reading portion(not shown). It should be noted that a function of the torque-limiterbetween the feed roller 5A and the separating roller 6 a, a change ofthe urging force N, and securing of the paper feed performance aresimilar to those of the first embodiment, so explanations thereof areomitted.

Here, also in this embodiment, the following two conditions aresatisfied.

1. The guide member 6 e is disposed so that the angle δ formed by theconveying direction of the sheet S and the guide member 6 e is equal toor smaller than 90°.

2. The guide surface angle θ is obtained as follows.$\theta < {\tan^{- 1}\left( \frac{1}{\mu\quad s} \right)}$

When the guide surface angle θ and the spring pressure Sp of theseparating roller are set to satisfy the above two conditions and aim toobtain the feed area in the relationship diagram between the rotationallowable tangential force and the urging force as shown in FIG. 7, thesheet feeding performance can be secured.

As described above, similarly to the first embodiment, the sheet feedingapparatus 3 in which the pickup roller 4 and the feed roller 5A areseparately arranged can be constituted with a width of the sheetseparating portion 6 substantially equal to that of the separatingroller while maintaining the sheet feeding performance, thereby makingit possible to minimize the space for the sheet separating portion 6 andthe space for the sheet feeding apparatus 3.

Next, a third embodiment of the present invention will be described.

FIG. 14 is a cross-sectional view for explaining a structure of anelectrophotographic printer as an example of an image forming apparatusincluding a sheet feeding apparatus according to the third embodiment ofthe present invention. It should be noted that in FIG. 14, the samereference symbols as those of FIG. 1 indicate the identical orcorresponding portions.

In FIG. 14, reference numeral 23 denotes a multiple sheet feedingportion serving as a sheet feeding apparatus provided at a right surfaceside of the apparatus main body 1. As shown in FIG. 15, the multiplesheet feeding portion 23 includes a multiple cover 22 c serving as thesheet stacking member which can be opened and closed with respect to theapparatus main body 1 and is arranged with a certain angle, a multiplesheet stacking plate 22 a for stacking the sheets S, the multiple sheetstacking plate 22 a being supported by the multiple cover 22 c in aswingable manner, and a multiple feed roller 25 serving as a sheetfeeding member.

The multiple sheet stacking plate 22 a is pressed upwardly by a multiplesheet stacking plate pressure spring 22 b serving as the urging memberfrom the back surface side (lower side in FIG. 15) of the multiple sheetstacking plate 22 a. A leading end of the uppermost sheet S1 stacked onthe multiple sheet stacking plate 22 a is pressed against the multiplefeed roller 25. The multiple cover 22 c fixes an angle made by themultiple sheet stacking plate 22 a with respect to the apparatus mainbody 1 and serves as a pedestal for the multiple sheet stacking platepressure spring 22 b.

The multiple feed roller 25 is contacted with the uppermost sheet S1stacked on the multiple sheet stacking plate 22 a, and at the same time,is contacted with a multiple separating roller 26 a provided to amultiple sheet separating portion 26 at a downstream side of the sheetconveying direction.

The multiple sheet separating portion 26 includes, in addition to themultiple separating roller 26 a, a torque-limiter 26 b coaxiallyconnected to the multiple sheet separating portion 26, a supportingmember 26 c for rotatably supporting the multiple separating roller 26 athrough the torque-limiter 26 b, a multiple separating roller spring 26d for pressing the supporting member 26 c from the back surface side ofthe supporting member 26 c (lower side in FIG. 15), and a guide member26 e including a guide surface having a predetermined angle with respectto a direction in which the multiple feed roller 25 is opposed to themultiple separating roller 26 a.

The multiple separating roller 26 a is rotatably supported by thesupporting member 26 c through the toque-limiter 26 b, and multipleseparating roller 26 a is slidably held by the guide member 26 e. Itshould be noted that a method of connecting the multiple separatingroller 26 a to the torque-limiter 26 b is similar to that of FIG. 3described above.

Further, an angle formed by the guide member 26 e and a direction alongwhich the multiple feed roller 25 is opposed to the multiple separatingroller 26 a and an angle formed by the guide member 26 e and a directionalong which the sheet S stacked on the multiple sheet stacking plate 22a collides with the multiple separating roller 26 a are similar to therelation among the feed roller 5, the separating roller 6 a, and theconveying sheet S, described in the first embodiment. Accordingly, afunction of the torque-limiter between the multiple feed roller 25 andthe multiple separating roller 26 a, a change in the urging force N, andsecuring of the paper feed performance are similar to those of the firstembodiment.

Therefore, also in this embodiment, the following two conditions aresatisfied.

1. The guide member 26 e is disposed so that the angle δ formed by theconveying direction of the sheet S and a guide surface of the guidemember 26 e is equal to or smaller than 90°.

2. The guide surface angle θ is obtained as follows.$\theta < {\tan^{- 1}\left( \frac{1}{\mu s} \right)}$

When the guide surface angle θ and the spring pressure Sp of theseparating roller are set to satisfy the above two conditions and aim toobtain the feed area in the relationship diagram between the rotationallowable tangential force and the urging force as shown in FIG. 7, thesheet feeding performance can be secured.

Therefore, even in the multiple sheet feeding portion 23 (sheet feedingapparatus) including the multiple cover 22 c (sheet stacking member) inwhich an angle for stacking sheets S is not substantially horizontalwith respect to the apparatus main body 1, and in the printer (imageforming apparatus) including such the multiple sheet feeding portion 23,when the guide member 26 e is arranged with a predetermined angle withrespect to a direction in which the multiple feed roller 25 is opposedto the multiple separating roller 26 a, and the multiple separatingroller 26 a is slidably supported by the guide member 26 e, it ispossible to constitute the multiple sheet separating portion 26 with awidth substantially equal to that of the separating roller whilemaintaining the sheet feeding performance, as described in the secondembodiment.

As described above, as long as the condition for the guide surface angleθ and the angle δ formed by the sheet conveying direction and the guidemember 26 e is satisfied in the multiple sheet feeding portion 23 (sheetfeeding apparatus), the angle with respect to the apparatus main body 1can be arbitrarily set. In other words, as long as the condition for theguide surface angle θ and the angle δ formed by the sheet conveyingdirection and the guide member 26 e is satisfied, even if the multiplecover 22 c is arranged perpendicularly to the apparatus main body 1, oreven if the feed roller is arranged below the sheet stacking portion(bottom separating method), it is possible to constitute the sheetseparating portion with a width substantially equal to that of theseparating roller while maintaining the sheet feeding performance.

Next, a fourth embodiment of the present invention will be described.

FIGS. 16A and 16B are diagrams for explaining a structure of a sheetfeeding apparatus according to the fourth embodiment of the presentinvention. It should be noted that in FIGS. 16A and 16B, the samereference symbols as those of FIG. 2 indicate the identical orcorresponding portions.

In FIG. 16A, reference numeral 86 denotes a supporting member forrotatably supporting the separating roller 6 a through thetorque-limiter 6 b, and reference numeral 87 denotes a guide member. Asshown in FIG. 16A, the supporting member 86 is provided with ribs 86 aand is shifted in a vertical direction while sliding the ribs 86 a alongthe guide member 87.

With such a structure in which the supporting member 86 is shifted in avertical direction while sliding the ribs 86 a along the guide member87, a contact area with the guide member 87 when the supporting member86 is shifted can be made small, thereby making it possible to suppressthe effect of a frictional resistance caused when the supporting member86 is shifted. As shown in FIG. 16B, even when ribs 89 a are provided toa guide member 89, it is possible to suppress the effect of thefrictional resistance caused when the supporting member 88 is shifted.

Further, as shown in FIG. 17A, it is possible to make a shape of each ofribs 90 a of a supporting member 90 to be arcuate. In this case, each ofthe ribs 90 a is in a point contact with a guide member 91, therebymaking it possible to suppress the sliding resistance caused when thesupporting member 90 is shifted. Alternatively, as shown in FIG. 17B, itis also possible to make a shape of each of ribs 93 a of a guide member93 to be arcuate. In this case, each of the ribs 93 a is in a pointcontact with the supporting member 92, thereby making it possible tosuppress the sliding resistance caused when the supporting member 92 isshifted.

Further, alternatively, as shown in FIG. 18A, supporting member 94 isprovided with rollers 94 a in a rotatable manner in place of the ribs,and the supporting member 94 is shifted in a vertical direction whilerotating the rollers 94 a along a guide member 95.

With such a structure in which the supporting member 94 is shifted in avertical direction while rotating the rollers 94 a along the guidemember 95, a contact area with the guide member 95 when the supportingmember 94 is shifted can be made small, thereby making it possible tosuppress the effect of a frictional resistance caused when thesupporting member 94 is shifted. As shown in FIG. 18B, even when rollers97 a are provided to a guide member 97 in a rotatable manner, it ispossible to suppress the effect of the frictional resistance caused whenthe supporting member 96 is shifted.

With any structure described with reference to FIGS. 16A and 16B, FIGS.17A and 17B, and FIGS. 18A and 18B, the effect of the present inventioncan be obtained. In addition, the structure is not limited to thosedescribed above, it is possible to adopt any structure as long as thefeed roller and the separating roller are slidably arranged with apredetermined angle with respect to a direction in which the feed rollerand the separating roller are opposed to each other.

In this embodiment, the description is made by taking as an example thecase where the present invention is applied to an image formingapparatus for forming an image on a sheet, but the present invention isnot limited thereto. It is also possible to apply the present inventionto an auto original feeder, which is an example of the sheet feedingapparatus used for the image reading apparatus or the like.

This application claims priority from Japanese Patent Application No.2005-128757 filed Apr. 26, 2005, which is hereby incorporated byreference herein.

1. A sheet feeding apparatus comprising: a sheet stacking memberconfigure to support sheets; a sheet feeding member configure to feedthe sheet from said sheet stacking member; a sheet separating portionincluding a separating roller provided so as to abut against said sheetfeeding member and a torque-limiter for allowing said separating rollerto rotate in association with said sheet feeding member when a rotationtorque larger than a predetermined torque is acted on said separatingroller and for preventing said separating roller from rotating inassociation with said sheet feeding member when the rotation torqueequal to or smaller than the predetermined torque is acted on saidseparating roller; a guide member configure to support said sheetseparating portion in a slidable manner and guiding said sheetseparating portion with a predetermined angle in a direction differentfrom a direction along which said sheet feeding member is opposed tosaid separating roller; and an urging member configure to urge saidsheet separating portion so that said separating roller is in pressurecontact with said sheet feeding member.
 2. A sheet feeding apparatusaccording to claim 1, wherein said sheet separating portion furthercomprising a supporting member for integrally supporting said separatingroller and said torque-limiter, and said supporting member is supportedby said guiding member in a slidable manner and is urged against saidsheet feeding member by said urging member.
 3. A sheet feeding apparatusaccording to claim 1, wherein an angle formed by the direction alongwhich said sheet feeding member is opposed to said separating roller anda direction along which said sheet separating portion slides is equal toor lower than $\tan^{- 1}\left( \frac{1}{\mu s} \right)$ where acoefficient of friction between sheets is given as μs.
 4. A sheetfeeding apparatus according to claim 1, wherein said guide membercomprises a flat guide surface, and said supporting member is supportedso that said supporting member is slide-able along said guide surface.5. A sheet feeding apparatus according to claim 4, wherein saidsupporting member comprises a rib, and said supporting member slideswhile bringing said rib into contact with said guide surface.
 6. A sheetfeeding apparatus according to claim 4, wherein said supporting membercomprises a rotatable roller, and said supporting member slides alongsaid guide surface while rotating said roller.
 7. A sheet feedingapparatus according to claim 1, wherein said guide member comprises arib, and said supporting member slides while being in contact with saidrib.
 8. A sheet feeding apparatus according to claim 1, wherein saidguide member comprises a rotatable roller, and said supporting memberslides while rotating said roller.
 9. An image forming apparatus,comprising: a sheet feeding apparatus as recited in claims 1; and animage forming portion for forming an image on a sheet fed from saidsheet feeding apparatus.